In my universe, humans nuked themselves in a war so thoroughly that the Earth has been rendered uninhabitable. As a result, they have to leave Earth and find a new home. They briefly tried to settle Mars, but were driven off by hostile native Martians they had neither the numbers nor the energy to fight. After this, they wandered the cosmos for another thousand years or so, inside a massive ship around the size of New York state. However, almost as soon as it arrived in this new galaxy, this ship was destroyed by a collision with a massive asteroid. As a result, almost half of the colonists went down with the ship, and those who survived were dispersed on a few dozen smaller ships that scattered themselves throughout this new galaxy, with each eventually finding and settling their own uninhabited new world.
-In this scenario, the humans have a technology level around the same as present-day, except with the spaceship capabilities mentioned above.
-The Martians were technologically inferior to the humans, only being able to beat them by way of sheer numbers.
Question: Once these humans all arrive at their new destinations, what would be a good reason for them to lose all their technology? For the purposes of the story, I need them to revert to Mesolithic levels relatively soon after they arrive.